The present invention generally discloses apparatus for measuring wellbores and more specifically involves calipers for measuring the diameter of a wellbore. It is known in the art to measure a wellbore with a caliper tool, which utilizes a sensing element such as a sliding potentiometer. The prior art devices utilize spring loaded arms to maintain contact with the wellbore wall and have connected to the sliding arms one or more elements of an electric potentiometer for determining the instantaneous borehole diameter.
The disadvantages suffered by this type of device include the exposure of the potentiometer wiring and parts to downhole conditions which lead to corrosion and failure of these parts. The prior art devices utilizing the potentiometer also suffer from the disadvantage of having internal sliding electronic components that wear mechanically and lead to inaccuracies and failure of the electrical system. The mechanical lead-ins to the potentiometer devices require structure for equalizing pressure between the inside of the tool and the wellbore. Also, pressure terminals through which the electrical and mechanical lead-ins pass must be sealed off against pressure and corrosive fluids.
The present invention disposes of these disadvantages by providing a magnetic sensing system to replace the complicated potentiometer system of the prior art devices. The magnetic sensing system is located behind a magnetic window which seals off and protects the electronics from well fluids and pressures. The only thing exposed to well fluids is the initiator magnet and the mechanical linkage connecting it to the caliper arms.